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Page 16 of 26 Wang et al. Soft Sci 2023;3:34 https://dx.doi.org/10.20517/ss.2023.25

characterizations indicated that the PVP was mainly located at the wall of micropores, and most Ag Se
2
grains have coherent interfaces [Figure 12]. Then, an optimal film exhibited a power factor as high as
2
~1,910 μW/mK (corresponding ZT ~1.1) at 300 K and excellent flexibility (only a 5.5% decline in power
factor after 1,000 times bending around a 4 mm radius rod). They also developed a one-pot method to
synthesize Ag Se powder instead of the Se NW template method. Then, the Ag Se film was prepared by
2
2
vacuum filtration, followed by hot pressing . Compared to the template method, the new approach is
[98]
much simpler, requires a significantly shorter time, has a higher yield, and is more environmentally friendly.
By adjusting the synthesis process of Ag Se powder, an optimal power factor could reach ~2,043 µW/mK .
2
2
Besides, TE properties of flexible Ag/Ag Se composite films with different Ag:Se ratios prepared by other
2
methods were also reported [99,100] , and the TE performances were listed in Table 3. Wu et al. fabricated
n-type Ag Se/Ag composite films using a similar method. They investigated the influence of Ag particle size
2
on the TE properties of the composite film through the adjustment of the sequence and reaction time of the
[101]
reducing agent L-ascorbic acid . After optimization, the flexible composite film showed a power factor as
high as 2,277.3 µW/mK . Palaporn et al. fabricated bacterial cellulose (BC)/Ag Se nanocomposite films by
2
2
blending BC with Ag Se powders, followed by vacuum filtration and hot pressing . The electrical
[102]
2
conductivity and Seebeck coefficient of the nanocomposites varied with the Ag/Se proportion due to the
changes in the carrier concentration and mobility. The highest power factor was 291 μW/mK at room
2
temperature.
Other inorganic TE materials
Besides Ag Se-based materials, researchers fully utilize the convenience and film-forming aiding properties
2
of the vacuum filtration method. Inorganic TE materials, such as Cu-Te, Cu-Se, Ag Te, Bi Te , and other
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2
3
inorganic semiconductors, are used to prepare flexible TE films using vacuum filtration. For example, Zhou
et al. prepared a free-standing flexible copper telluride NW/PVDF (Cu Te NWs/PVDF = 2:1) thin film
1.75
[105]
using a five-step vacuum filtration process [Figure 13] . By burying the Cu Te NWs into the PVDF
1.75
matrix, the flexible fabric exhibits a Seebeck coefficient and electric conductivity of 9.6 μV/K and 2,490 S/cm
at room temperature, respectively, resulting in a power factor of 23 μW/mK . Pammi et al. reported Cu Se
2
2-x
NW/PVDF composite flexible thin films, which achieved a power factor of 105.32 μW/mK 2[106] . Notably, the
mechanical hot-pressing transfer process followed by vacuum filtration was found to have increased
conductance due to a reduction in the junction resistance and to an interconnection density between NWs
and densification compared to various fabrication methods. CuI/nylon composite films with a Seebeck
[107]
coefficient as high as 600 µV/K were also prepared . Although the low electrical conductivity limited its
TE performance, the quick voltage response under temperature gradient showed the potential application in
wearable thermal sensors.
To improve the electrical conductivity of and maintain the initial morphology of the Ag Te NWs, Zeng
2
et al. reported an approach to welding Ag Te NWs at room temperature to enhance their contacts by the
2
combination of vacuum filtration and drop-coating methods . Under compressive stress and atomic
[108]
diffusion, a diffusion weld is generated at the intersection of the NWs to form a room-temperature welded
Ag Te NW film. The welded Ag Te NW film showed a carrier concentration of about one-half that of the
2
2
typical Ag Te NW film and a carrier mobility of four times larger than that of vacuum filtration-assisted
2
Ag Te NW film (with a loose connection). Finally, the welded Ag Te NW film showed a power factor of
2
2
359.76 μW/mK at 420 K, about three times larger than the vacuum-filtrated Ag Te NW film. Yu et al.
2
2
fabricated a robust, flexible Ag Te NW film by in situ chemical transformation and vacuum-assisted
x
filtration . Then, its TE performance was further optimized by adjusting the Ag/Te ratio and
[109]
post-treatment pressures/pressing temperature. And the Ag Te NW film displayed a tensile strength of
x
2
~78.4 MPa with a power factor of 48.9 μW/mK at room temperature. Besides, the TE properties of cellulose
[111]
[110]
nanofiber (CNF)/Bi Te composite films , paper matrix-based Bi Te and Sb Te composite films , Se
3
2
2
3
2
3